We examined the interaction of GABA and the competitive inhibitor SR95531 at human alpha(1)beta(1)gamma(2S) and alpha(1)beta(1) GABA(A) receptors expressed in Sf9 cells. The efficacy and potency of inhibition depended on the relative timing of the GABA and SR95531 applications. In saturating (10 mM) GABA, the half-inhibitory concentrations of SR95531 (IC50) when coapplied with GABA to alpha(1)beta(1)gamma(2S) or alpha(1)beta(1) receptors were 49 and 210 mu M for the peak and 18 and 130 mu M for the plateau current, respectively. Our data are explained by an inhibition mechanism in which SR95531 and GABA bind to two sites on the receptor where the binding of GABA allows channel opening but SR95531 does not. The SR95531 affinity for both receptor types was similar to 200 nM and the binding rate was found to be 10-fold faster than that for GABA. The dual binding-site model gives insights into the differential effects of GABA and SR95531 on the peak and plateau currents. The model predicts the effect of SR95531 on GABA currents in the synapse (GABA concentration similar to mM) and at extrasynaptic (GABA concentration <= mu M) sites. The IC50 (50-100 nM) for the synaptic response to SR95531 was insensitive to the GABA affinity of the receptors whereas the IC50 (50-800 nM) for extrasynaptic inhibition correlated with the GABA affinity.
Journal of Neurochemistry Vol. 94, no. 2, p. 491-501